Nowadays, various transition metal complexes each composed of a transition metal and a ligand are aggressively used as a catalyst in organic synthesis reactions. It is known that not only the type of the transition metal but also the ligand, i.e., an organic compound containing a group having a lone electron pair (coordinating group) capable of coordinating to a metal species, plays a very important role as a factor of exhibiting the performance and activity of the catalyst above.
Of these ligands, an organic compound having four coordinating groups (tetradentate ligand) forms three chelate rings at the time of coordination and therefore has a characteristic such that its metal complex is highly stabilized. Furthermore, in a metal complex having a regular octahedral structure, the tetradentate ligand can be coordinated not only in the trans fashion but also in the cis-α/cis-β fashion and thus can induce a new asymmetric environment in the metal center.
In this way, the tetradentate ligand exhibits an interesting coordination behavior and therefore, occupies an important position in the field of complex chemistry, catalyst chemistry, organic synthesis chemistry, etc., and researches and developments thereof are still actively conducted at present. The tetradentate ligand structures reported are so far extended from a simple one capable of being synthesized in a short process to a complicated one requiring a multi-stage reaction, but from the industrial viewpoint, a tetradentate ligand with a simpler structure enabling easy large-scale synthesis is preferred.
As an example of such a tetradentate ligand, a dehydrative condensate of 2-diphenylphosphinobenzaldehyde and an ethylenediamine derivative is known to behave as a PNNP (phosphorus-nitrogen-nitrogen-phosphorus) tetradentate ligand for metal species. It has been reported that a ruthenium complex of the PNNP tetradentate ligand above exhibits good catalytic activity, for example, in a hydrogenation reaction of esters (Patent Document 1 and Non-Patent Document 1).
Furthermore, in recent years, it has been reported that a dehydrative condensate of an ethylenediamine derivative and 2-alkylthiobenzaldehyde capable of being synthesized from inexpensively available 2-nitrobenzaldehyde also functions as an SNNS (sulfur-nitrogen-nitrogen-sulfur) tetradentate ligand, and that a ruthenium complex of this SNNS tetradentate ligand can be an excellent catalyst in an asymmetric hydrogenation reaction of ketones (Non-Patent Document 2).